This invention relates to a circuit arrangement for operating a discharge lamp by means of a high-frequency current, comprising
switching means for generating the high-frequency current from a supply voltage, PA1 a modulator for the substantially square-wave modulation of the amplitude of the high-frequency current with a modulation frequency f, and PA1 means M for limiting the interference with infrared systems caused by the discharge lamp.
Such a circuit arrangement is described in European Patent Application, not previously published, and which corresponds to U.S. Pat. No. 5,243,261 (Sep. 7, 1993).
The circuit arrangement is suitable for operating discharge lamps, in particular low-pressure discharge lamps, more particularly electrodeless low-pressure discharge lamps.
During lamp operation by means of the circuit arrangement described therein, a substantially square-wave modulated high-frequency voltage is present across the discharge lamp, to be referred to as the lamp hereinafter. The frequency and phase of the substantially square-wave modulation of the high-frequency voltage across the lamp are the same as the frequency and phase of the substantially square-wave modulation of the high-frequency current. At the start of each square wave of the substantially square-wave modulated high-frequency voltage, the lamp is re-ignited by the high-frequency voltage which then acts as a re-ignition voltage. After this the amplitude of the high-frequency voltage drops to a substantially constant lamp-dependent value owing to the fact that the lamp becomes conducting, and the high-frequency voltage causes the high-frequency current to flow through the lamp during the square wave. In the remaining portion of each cycle of the substantially square-wave modulation of the high-frequency voltage, practically no voltage is present across the lamp and practically no current flows through the lamp. The lamp is dimmed in that a duty cycle .delta. of the substantially square-wave modulation of the high-frequency voltage is adjusted. It was found that the luminous efficacy is comparatively high and substantially independent of the luminous flux with this dimming method.
The means M in the circuit arrangement described in the European Patent Application and the corresponding U.S. Pat. No. 5,243,261 comprise means for limiting the amplitude of the re-ignition voltage with which the discharge lamp is re-ignited at the start of each square wave. The circuit arrangement may also be provided with means for limiting the speed with which the amplitude of the high-frequency current drops at the end of each square wave of the square-wave modulation. These measures each produce a considerable reduction in the interference with infrared systems caused by the discharge lamp. A disadvantage of the known circuit arrangement, even if provided with the two measures indicated above, however, is that a lamp operated by means of this circuit arrangement still causes a certain amount of interference with infrared systems.